4R-{(1R,4S,6S,9R)-4-azidomethyl-7-N-(3-tert-butoxycarbonylaminopropyl)-3-N-(2-cyanoethyl)-3,7-diaza-8-oxa-2-oxobicyclo[4.3.0]nonan-9-yl}-2,3-di-O-tert-butyldimethylsilyl-1-(uracil-1-yl)-β-D-erythrofuranose